Implantable cardiac defibrillators are used to treat patients suffering from ventricular fibrillation, a chaotic heart rhythm that can quickly result in death if not corrected. In operation, the defibrillator device continuously monitors the electrical activity of the heart of the patient, detects ventricular fibrillation, and in response to that detection, delivers appropriate shocks to restore a normal heart rhythm. Shocks as large as 30-35 joules may be needed. Shocks are delivered from capacitors capable of providing that amount of energy to the patient in a fraction of a second. To provide timely therapy to the patient after the detection of ventricular fibrillation, it is necessary to charge the capacitors with the required amount of energy in only a few seconds. Thus, the power source must have a high rate capability to provide the necessary charge to the capacitors, possess low self-discharge to have a useful life of many months, and must be highly reliable to provide an urgently needed therapy whenever necessary. In addition, since cardiac defibrillators are implanted, the battery must be able to supply energy from a minimum packaged volume.
One battery suitable for defibrillator use includes silver vanadium oxide as a cathode material as disclosed in U.S. Pat. Nos. 4,310,609 or 4,391,729 issued to Liang et al or U.S. Pat. No. 5,221,453 issued to Crespi. The cathode materials described in the foregoing Liang and Crespi patents can find application in the batteries or cells disclosed in U.S. Pat. Nos. 5,458,997; 5,312,458; 5,298,349; 5,250,373; 5,147,737; 5,114,811; 5,114,810; 4,964,877; and 4,830,840. All the foregoing patents are hereby incorporated by reference herein in their respective entireties.
As disclosed in some of the foregoing patents, the anode material of the battery is lithium and the reactive cathode material is silver vanadium oxide. The electrolyte for a lithium battery or cell is a liquid organic type which comprises a lithium salt in combination with an organic solvent.
Organic solvents known for use in lithium cells can be, for example, 3-methyl-2-oxazolidone, sulfolane, tetrahydrofuran, methyl-substituted tetrahydrofuran, 1,3-dioxolane, propylene carbonate (PC), ethylene carbonate, gamma-butyrolactone, ethylene glycol sulfite, dimethylsulfite, dimethyl sulfoxide or mixtures thereof and also, for example, low viscosity cosolvents such as tetrahydrofuran (THF), methyl-substituted tetrahydrofuran(Met-THF), dioxolane (DIOX), dimethoxyethane (DME), dimethyl isoxazole (DMI), diethyl carbonate(DEC), ethylene glycol sulfite (EGS), dioxane, dimethyl sulfite (DMS) or the like. The ionizing solute for lithium cells can be a simple or double salt or mixtures thereof, as for example, LiCF.sub.3 SO.sub.3, LiBF.sub.4, LiAsF.sub.6, LiPF.sub.6 and LiCIO.sub.4 which produce an ionically conductive solution when dissolved in one or more solvents. An organic solvent composition commonly used for lithium/silver vanadium oxide cells has been a mixture of propylene carbonate and 1,2-dimethoxyethane in a 50/50 ratio.
The selection of the particular solvent components and acceptable ratios of the solvent components can prove to be a difficult task even if each component is individually well known. Typically a solvent component may be selected for its dielectric constant, for its capabilities as a solvent for the particular solute material, for its viscosity or for other properties which may be unique to a particular cell. For example, since 1,2-dimethoxyethane has a low viscosity and a low dielectric constant, it is commonly mixed with another polar aprotic solvent having a higher dielectric constant (e.g., propylene carbonate, ethylene carbonate, or gamma-butyrolactone) for use in practical lithium cells and batteries. Such a solvent mixture possesses better properties for the ionization of lithium salts and wetting of the electrode and separator surfaces than either of the component solvents alone.
Electrolytes have also been indicated to be suitable for use in lithium cells with three solvent components. For example, U.S. Pat. No. 4,129,691 issued to Broussely, and hereby incorporated by reference herein in its entirety, discloses an electrolyte for use in lithium/cupric oxide or lithium/ferrous sulfide primary cells which is made from a mixture of three organic solvents and an alkaline solute. The first solvent is chosen to have a dielectric constant equal to or greater than 35 (e.g. propylene carbonate), the second solvent is a linear polyether with its ether functional groups in the gamma position (e.g. 1,2-dimethoxyethane) and the third solvent has a high solvation power for dissolving large quantities of the alkaline salt (e.g. 1,3-dioxolane).
In lithium/silver vanadium oxide cells, it has been noted that the cell tends to increase in resistance in a roughly time-dependent manner after the battery is discharged to a second voltage plateau. This means that on long-term discharge, these cells can develop high resistance that impairs their ability to charge the capacitors of a defibrillator in a timely manner and therefore renders much of the capacity of the cell unavailable for long term use in an implantable defibrillator. Further, the end of service determination in these cells is complicated by the variable nature of the resistance buildup. In an experiment which substituted ethylene carbonate for propylene carbonate, it was found that the irreversible resistance was much worse with the ethylene carbonate. This is contrary to expectation since ethylene carbonate has a higher dielectric constant than propylene carbonate so that the solvent with ethylene carbonate should have reduced resistance for the cell. Accordingly, it is believed that the solubility of the silver vanadium oxide cathode material in the electrolyte solvent contributes to the build-up of resistance over time.
It is therefore an object of the present invention to provide a high current rate capability lithium/silver vanadium oxide battery having a reduced resistance at the second voltage plateau.
It is also an object of the present invention to provide an electrolyte for a lithium/silver vanadium oxide battery which provides improved discharge characteristics for the battery.